Ultralong lifetime for fully photogenerated spin-polarized current in two-dimensional ferromagnetic/nonmagnetic semiconductor heterostructures

Photogenerated fully spin-polarized current has attracted considerable attention in spintronics owing to its high efficiency for information processing but low energy consumption. However, the short spin carrier lifetime of the primary materials significantly impedes the practical application of spin-polarized current and presents a major challenge. Here, on the basis of density functional theory and nonadiabatic molecular dynamics simulations, we design two prototypes of two-dimensional (2D) ferromagnetic/nonmagnetic semiconductor heterostructures ($\mathrm{Cr}{\mathrm{I}}_3/\mathrm{Mo}{\mathrm{Se}}_2$ and $\mathrm{Cr}{\mathrm{Br}}_3/\mathrm{Mo}{\mathrm{S}}_2$) with demonstrated ultralong photogenerated spin carrier lifetime. It is shown that the photogenerated spin holes can be effectively injected from the ferromagnetic layer to the nonmagnetic layer, and fully spin-polarized electrons will be confined in the ferromagnetic layer due to their type-II spin-polarized channel and large spin-flip gap. More surprisingly, the photogenerated spin carrier lifetime of these heterostructures is up to the time scale of nanoseconds. Such an ultralong spin carrier lifetime is mainly originated from small nonadiabatic coupling induced by weak spin electron-phonon interaction and slow nuclear velocity. Our results provide insights into the design of 2D semiconductor spintronic devices with an ultralong fully spin-polarized current lifetime.

Figure 1

(a) Schematic diagrams of three favorable conditions for generating and improving photogenerated fully spin-polarized carrier lifetime: (1) large spin-flip gap, (2) suitable band gap, and (3) type-II band alignment. FM and NM represent ferromagnetic and nonmagnetic semiconductor, respectively. (b) Top and side views of $\mathrm{Cr}{X}_3$ ($X=\mathrm{I}$, Br) and $\mathrm{Mo}{Y}_2$ ($Y=\mathrm{S}$, Se) monolayers, and (c) $\mathrm{Cr}{X}_3/\mathrm{Mo}{Y}_2$ heterostructures with AB1, AB2, and AB3 stacking configurations. The pink, green, blue, and violet balls represent Mo, $Y$ (S, Se), Cr, and $X$ (I, Br) atoms, respectively. The black vertical lines are marked to identify the different stacking configurations.

Figure 2

(a) Band structure of $\mathrm{Cr}{\mathrm{I}}_3/\mathrm{Mo}{\mathrm{Se}}_2$ (left) and $\mathrm{Cr}{\mathrm{Br}}_3/\mathrm{Mo}{\mathrm{S}}_2$ (right) heterostructures. The blue and red spheres represent spin-up and spin-down channels for $\mathrm{Cr}{\mathrm{I}}_3$ and $\mathrm{Cr}{\mathrm{Br}}_3$, respectively. (b), (c) Corresponding charge density of CBM and VBM of $\mathrm{Cr}{\mathrm{I}}_3/\mathrm{Mo}{\mathrm{Se}}_2$ and $\mathrm{Cr}{\mathrm{Br}}_3/\mathrm{Mo}{\mathrm{S}}_2$ heterostructures, respectively.

Figure 3

(a) Schematic image of photogenerated spin carrier dynamics processes: (1) spin hole injection, (2) interlayer spin $e$-h recombination, and (3) intralayer spin $e$-h recombination. (b), (d) State population of the above three dynamics processes in $\mathrm{Cr}{\mathrm{I}}_3/\mathrm{Mo}{\mathrm{Se}}_2$ and $\mathrm{Cr}{\mathrm{Br}}_3/\mathrm{Mo}{\mathrm{S}}_2$ heterostructures. All data are fitted by the exponential function $f\left(t\right)=\exp \left(-t/\tau \right)$. (c), (e) The average nonadiabatic coupling (NAC) value between the donor and acceptor states. The larger the NAC, the stronger the coupling.

Figure 4

Spectral densities characterizing phonon modes involved in (a) spin hole injection and (b) interlayer spin $e$-h recombination of $\mathrm{Cr}{\mathrm{I}}_3/\mathrm{Mo}{\mathrm{Se}}_2$ and $\mathrm{Cr}{\mathrm{Br}}_3/\mathrm{Mo}{\mathrm{S}}_2$ heterostructures and (c) intralayer spin $e$-h recombination of $\mathrm{Cr}{\mathrm{I}}_3$ and $\mathrm{Cr}{\mathrm{Br}}_3$ monolayers.

Figure 5

Pure-dephasing functions of (a) interlayer spin $e$-h recombination and (b) intralayer spin $e$-h recombination. The time data are fitted by the Gaussian function $f\left(t\right)=\exp \left(-0.5{\left(t/\tau \right)}^2\right)$.